1. Field of the Invention
The present invention relates to a tunable laser including a birefringence filter and a photoacoustic device including the tunable laser.
2. Description of the Related Art
Tunable laser light sources have been widely used in various fields including medical and measurement fields and actively been studied. With the photoacoustic tomography (PAT), distribution of light absorption, that is, distribution of objects in a living body can be imaged by, for example, irradiating the living body with light of a wavelength band in the range of 700 to 900 nm, which is less likely to be absorbed by water, and detecting an acoustic wave that occurs as a result of the light being absorbed by the objects in the living body. A tunable laser light source having the following configuration is known. The light source includes a gain medium having a stimulated-emission cross-section in a desired waveband and switches the oscillation wavelength using a birefringence filter.
FIG. 11 schematically illustrates a configuration of a tunable laser light source including an existing birefringence filter. A partially reflecting mirror 105 and a highly reflecting mirror 104 form a resonator. Part of light emitted from a gain medium 109 excited by an excitation device 110 is caused to oscillate by the resonator and the gain medium 109. Part of the oscillating light (beam) passes through the partially reflecting mirror 105 and is taken to the outside as an output.
The birefringence filter includes one or more birefringent plates superposed parallel to one another such that their crystal principal dielectric axes (principal axes of refraction ellipsoids) coincide with one another. The birefringence filter is arranged such that an angle (insertion angle) θ formed by an optical axis 107 of a beam and the normal to the birefringence filter coincides with Brewster angle θB of the birefringent plates.
At this time, the reflectivity of p-polarized light 106 that is to be reflected off the surfaces of the birefringent plates is zero, and thus the transmission of the p-polarized light 106 (which is parallel to the surface of FIG. 11) that passes through the birefringence filter at the transmission peak of the birefringence filter is approximately one (100%). On the other hand, s-polarized light 108 is reflected off the surfaces of the birefringent plates and thus is lost through the reflection to a larger extent than the p-polarized light 106, whereby the s-polarized light 108 does not substantially contribute to oscillation.
The following description is given considering only a component of light incident on the birefringence filter as p-polarized light (having an electric field intensity Epin) and a component (having an electric field intensity Epout) that transmits as p-polarized light among transmission light components.
The transmission and transmission spectrum of the birefringence filter are defined by the electric field intensity ratio (Epout)2/(Epin)2. In the transmission spectrum of the birefringence filter, the peak of the transmission at which the transmission is approximately one is called a transmission peak.
The oscillation wavelength is switched by changing the wavelength at the transmission peak of the birefringence filter. Specifically, the wavelength at the transmission peak is shifted by rotating the birefringence filter around the normal (having a rotation angle φ in FIG. 11) to the optical surface of the birefringence filter. In the configuration illustrated in FIG. 11, the transmission at the shifted transmission peak can be maintained at one. The details of the configuration of the birefringence filter and a method of switching the oscillation wavelength are described in “the birefringent filter” by John W. EVANS, in Journal of the Optical Society of America, vol. 39, issue 3, pp. 229-237 (1949) and Japanese Patent Laid-Open No. 2002-232047.
FIG. 12 and FIG. 13 are graphs for describing the number of birefringent plates appropriate for a tunable laser light source.
FIG. 12 illustrates a transmission spectrum of a birefringence filter including one birefringent plate. The transmission spectrum has multiple peaks at which the transmission is one. An interval Δλ between the transmission peaks is substantially in inverse proportion to the thickness d of the birefringent plate and the full width δλ at half maximum corresponding to the transmission peak is approximately Δλ/2. Generally, the thickness d of the birefringent plate is selected such that the interval Δλ is larger than the width of a wavelength variable range and thus the full width δλ at half maximum corresponding to the transmission peak is larger than the half of the width of the wavelength variable range.
Meanwhile, the tunable laser light source is required to have a sufficiently smaller full width δλ at half maximum corresponding to the transmission peak than the wavelength variable range in order to stabilize the oscillation wavelength. Thus, the configuration including one birefringent plate illustrated in FIG. 13 cannot maintain a stable oscillation wavelength.
FIG. 13 illustrates transmission spectra of the birefringence filters respectively including two, three, and four birefringent plates. The thicknesses of the second, third, and fourth birefringent plates are respectively two, three, and four times the thickness d of the thinnest birefringent plate. In this configuration, an interval Δλ between transmission peaks is substantially in inverse proportional to the thickness d of the thinnest birefringent plate.
In the case where the birefringence filter includes two birefringent plates, the full width δλ at half maximum corresponding to the transmission peak is approximately ⅙ the interval Δλ between the transmission peaks. Thus, the thickness d of the birefringent plate can be selected such that the interval Δλ between the transmission peaks is wider than the width of the wavelength variable range and the full width δλ at half maximum corresponding to the transmission peak is sufficiently smaller than the width of the wavelength variable range, thereby enabling stable oscillation switching. As illustrated in FIG. 13, an increase in number of birefringent plates enables further reduction in full width δλ at half maximum corresponding to the transmission peaks.
As the number of birefringent plates increases, however, an adjustment of crystal principal axes of birefringent plates becomes more difficult, thereby increasing the production cost of birefringent plates. For this reason, in a tunable laser light source, in which having a characteristic of stable wavelength switching regardless of a spectrum width of oscillating beams is regarded as important, a birefringence filter including two birefringent plates is preferred.